Conjugated Planar Molecules

We have seen that in molecules one can consider two families of wave functions the localizable a family and the non-localizable n family. The dipole moment of the molecule is the sum of the dipole moments of the various a bonds ( r moment) and of the n moment. Of course, the n moment could also be partitioned into bond contributions. But the values that would be obtained would not have the quasiinvariant character of the a bonds. For instance, in azulene the various CC bonds would have highly different moments. On the other hand, the partitioning into bond moments is of interest only when the moments obtained have a sufficient degree of transferability. We are interested here only in the a moment. 

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Cyclic, fully conjugated, planar molecules with 4n + 2 pi electrons (n = any integer including zero) are aromatic. 

The criteria for an antiaromatic compound can be generalized in a similar manner. The requisite number of electrons to have the HOMOs half filled is a multiple of four (an even number of pairs). Therefore, cyclic, fully conjugated, planar molecules with 4n pi electrons are antiaromatic. Some of the possible numbers of electrons are as follows ... 

Tetracyanoethylene (TCNQ) shown below is a highly conjugated planar molecule. Its ir-type LUMO (represented below) lies at a rather low energy so that the compound can be easily reduced by one electron, giving rise to the stable radical anion TCNQ-. 

FIGURE 18 6 Acrolein w (H2C=CHCH=0) is a planar molecule Oxygen and each carbon IS sp hybridized and each contributes one elec tron to a conjugated tt elec tron system analogous to that of 1 3 butadiene... 

Discernible associative character is operative for divalent 3t5 ions through manganese and the trivalent ions through iron, as is evident from the volumes of activation in Table 4. However, deprotonation of a water molecule enhances the reaction rates by utilising a conjugate base 7T- donation dissociative pathway. As can be seen from Table 4, there is a change in sign of the volume of activation AH. Four-coordinate square-planar molecules also show associative behavior in their reactions. 

Cyclazines are conjugated cyclic molecules in which planarity is secured by three covalent bonds to a central nitrogen atom. They differ in the size of the individual ring units. In the nomenclature of cyclazines, the name is preceded by the number of jt-centers between the points of attachment to the nitrogen which are arranged by increasing number and put in brackets, e.g. [3.3.3]cyclazine (1) and [2.3.4]cyclazine (2). 

The example of COj discussed previously, which has no vibrations which are active in both the Raman and infrared spectra, is an illustration of the Principle of Mutual Exclusion For a centrosymmetric molecule every Raman active vibration is inactive in the infrared and any infrared active vibration is inactive in the Raman spectrum. A centrosymmetric molecule is one which possesses a center of symmetry. A center of symmetry is a point in a molecule about which the atoms are arranged in conjugate pairs. That is, taking the center of inversion as the origin (0, 0, 0), for every atom positioned at (au, yi, z ) there will be an identical atom at (-a ,-, —y%, —z,). A square planar molecule XY4 has a center of symmetry at atom X, whereas a trigonal planar molecule XYS does not possess a center of symmetry. 

Other approximate, more empirical methods are the extended Huckel 31> and hybrid-based Hiickel 32. 3> approaches. In these methods the electron repulsion is not taken into account explicitly. These are extensions of the early Huckel molecular orbitals 4> which have successfully been used in the n electron system of planar molecules. On account of the simplest feature of calculation, the Hiickel method has made possible the first quantum mechanical interpretation of the classical electronic theory of organic chemistry and has given a reasonable explanation for the chemical reactivity of sizable conjugated molecules. 

Free pyrimidines and purines are weakly basic compounds and are thus called bases. They have a variety of chemical properties that affect the structure, and ultimately the function, of nucleic acids. The purines and pyrimidines common in DNA and RNA are highly conjugated molecules (Fig. 8-2), a property with important consequences for the structure, electron distribution, and fight absorption of nucleic acids. Resonance among atoms in the ring gives most of the bonds partial double-bond character. One result is that pyrimidines are planar molecules purines are very nearly... 

The stability of a molecule based on its filled and empty MOs, and its delocalization energy or resonance energy, based on a comparison of its total n-energy with that of a reference system. The pattern of filled and empty MOs led to Hiickel s rule (the An I 2 rule) which says that planar molecules with completely conjugated p orbitals containing An I 2 electrons should be aromatic. 

Only little has been reported on second-order hyperpolarizabilities yin two-di-mensionally conjugated molecules. Planar systems as e.g. phthalocyanines have been studied for two photon absorption which is proportional to the imaginary part of the nonlinearity y. For planar molecules with a three-fold symmetry, the importance of charge transfer from the periphery to the center of the molecule in order to realize large nonlinearities ywas reported [65]. Off-resonant DFWM experiments revealed promising third-order nonlinearities in two-dimensional phenylethynyl substituted benzene derivatives [66]. Recently, the advantage of two-dimensional conjugation to increase the values of the first-order hyperpolarizability p has also been pointed out [67-69]. 

Analysis of the data indicates the bond lengths are in the normal range for this essentially planar molecule and the benzene portion exhibits considerable tr-conjugation. The best resonance structure for the thiadiazole is a partial double bond for S—C(7a) and S—N(2), and a nearly complete double bond for N(2)—N(3) and C(7a)—C(3a). One might expect the bond angles and lengths for thiadiazole to vary if it were not fused to the benzene ring. 

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